1. Field of the Invention
This invention relates to apparatus for use in cementing of the outer casing annulus of a wellbore, and more particularly, to an apparatus which carries an accelerating fluid to the proper injection point or maintains the fluid at the proper injection point until the apparatus is activated.
2. Description of the Prior Art
Cementing of casing into a wellbore is well known in the art. Cement is pumped into the well casing through a casing shoe or a cementing valve installed in the casing so that the cement is positioned in the desired zone. Depending upon conditions, it may be necessary to mix additives with the cement to retard setting time, accelerate setting time, control fluid loss in the cement, gel the cement, reduce the slurry density, lighten the slurry or increase its weight, increase its mechanical strength when set, reduce the effect of mud on the cement, improve its bonding, or to effect more than one of the above purposes, as well as others. To do this, additives are mixed with the cement slurry.
Additives have been mixed on the surface and then pumped with the cement into the well. Alternatively, a portion of the cement may be pumped, additive pumped after that, and more cement pumped, etc. For example, in order to accelerate the setting up of a cement column in a subterranean well, it is necessary to inject certain chemicals, such as accelerators, into the cement slurry at the proper time, at the proper place and in the proper proportions. This procedure has the obvious drawback that an additive starts working as soon as it contacts the cement, and it is never certain that the mixed cement and additive will reach the desired location at the correct time which may result in the cement setting up too soon or too late.
Since the cement slurry must remain pumpable for a specified period of time, it is desirable to inject the chemicals into the cement slurry downhole rather than at the surface during mixing. This allows the accelerator to act only when desired and not set up the cement too soon. Devices for carrying out such injection have been developed. One such device is shown in U.S. Pat. No. 4,361,187 which discloses a downhole mixing valve for use in cementing, fracturing or other treatment of a well. This valve is generally mounted on a pipe string which is run into the well casing. This has worked well, but it does require an additional trip with the pipe string which increases costs and the time of the cementing job.
The apparatus of the present invention solves the problems of the previous devices in that it includes a mechanism for either carrying the accelerator to the proper injection point or maintaining the accelerator at the proper injection point until the device is activated. The accelerator may then be injected into the fluid without need of an additional trip with a pipe string.